net/mlx5: E-Switch, Err when retrieving steering name-space fails

Make sure to return error when we failed retrieving the FDB steering
name space. Also, while around, correctly print the error when mode
change revert fails in the warning message.

Signed-off-by: Or Gerlitz <ogerlitz@mellanox.com>
Reported-by: Leon Romanovsky <leonro@mellanox.com>
Reviewed-by: Roi Dayan <roid@mellanox.com>
Signed-off-by: Saeed Mahameed <saeedm@mellanox.com>